Optimization of the filters thickness for the SINRD technique applied to spent fuel verification

One of the Non-Destructive Assays (NDA) methods that is under research at SCK•CEN for the safeguards verification of spent fuel is Self-Interrogation Neutron Resonance Densitometry (SINRD). This technique relies on the absorption of neutrons around the 239Pu resonance at 0.3 eV to directly quantify the residual mass of this nuclide in the fuel assembly. The performance of this NDA technique is mainly investigated with Monte Carlo simulations. Our approach for the SINRD technique aims at measuring the neutron fluence in the guide tubes of a PWR fuel assembly by introducing a set of fission chambers in these positions. Preliminary studies suggested that it is possible to correlate the neutron fluence in different energy windows with the residual 239Pu content. Since SINRD is focused on measuring the neutron fluence in a specific energy window, and since neutron detectors cannot directly provide information about the incident neutron energy, the proposed way to select the energy region around 0.3 eV is by wrapping layers of specific materials around the detector: one that will count neutrons with energy higher than 0.2 eV and one with cut- off above the resonance (e.g. 0.4 eV). We foresee the use of Gd and Cd filters to define the sensitive energy window. This work reports about the simulations that were carried out to optimize the filter thicknesses in order to increase the sensitivity to 239Pu of the proposed method in the envisaged geometry. The sensitivity as a function of the filters thickness was determined and optimal measurement conditions were determined.